What is useState?

The useState hook is the most popular and widely used hook in React. It lets you have state in a functional component. Before the useState hook came out, (React 16.8 🎉), you could only have state in class based components 😭.

How do you use it?

useState is a function.

useState()

That function returns 2 things: the state, and a function to update that state. You typically access those through array destructuring.

const [state, setState] = useState()

You can pass that function an initial state.

const [state, setState] = useState(initialState)

That initial state can be anything: a number, a boolean, a string... It can even be a function that returns the initial state.

💡 That function would only be called on the
initial render. Not every time the state is updated or on
every re-render. So having a more expensive function here is
ok if you need it.

We'll start off with the classic counter example. So the initial state will be 0.

const [state, setState] = useState(0)

It's good practice to name the state (and setState) something that refers to that state. Since we're using our state as a counter, we'll name our state count (and our updater function setCount).

const [count, setCount] = useState(0)

Accessing the State

We can access our state in our component simply by using the const variable we assigned to our state. In our case, count.

<p>Our count number is: {count}</p>

This will render:

Updating the State

Now we'll create a button that when clicked, it will update the state.

<button onClick={() => setCount(count + 1)}>count it!</button>

In this example, we're inserting the code to update the state right inside setCount (count + 1). We could also call a function that has more logic, then update the state in that function.

const countHandler = () => {
  // more logic could go here 
  //(reach out to an api, more calculations, etc..)
  const updatedCount = count + 1;
  setCount(updatedCount)
}
<button onClick={countHandler}>count it!</button>

This would have the same behavior.

Conclusion

useState can be used for all sorts of things: grabbing a user's input from a form, toggling a boolean for a mobile sidebar, or even having an array of blog posts you retrieved from a database and mapping over them to display each post.

I go over a lot more examples in my YouTube video about useState here, and I focus specifically on form handling here.

Hope this helped someone and thank you for reading!

-Andrew